Extractability and Fractionation of Phosphorus in Different Biochars.

Biochar as a promising soil amendment may be a consideralbe source of phosphorus (P) to plants and the environment. To predict the release potential and bioavailability of P in biochar, chemical and spectroscopic analyses were conducted on biochar P extractability and species distribution. Poultry litter biochars (PLC) and cottonseed meal biochars (CMC) produced at 300, 350, 400, 450, 500, 550, and 600 degree Celcius pyrolysis temperatures were analyzed for the contents of total P, inorganic P, organic P, Mehlich-3 extractable P, Bray-1 extractable P, Olsen extractable P, and iron oxide-impregnated filter paper extractable P. The total biochar P was fractionated into water-soluble inorganic & organic, NaHCO₃-extractable inorganic & organic, NaOH-extractable inorganic & organic, HCl-extractable, and residual fractions by sequential extraction methods. The biochars were further characterized using solid-state ³¹P NMR techniques for existing P species. The results suggest that the P extractability and fractionation of biochar vary significantly with the feedstock and pyrolysis temperature. Conversion to biochar decreases the P extractability if the feedstock contains high levels of multivalent metal salts; the P extractability of biochar further decreases with elevating the pyrolysis temperature. The water-soluble and NaHCO3-extractable P fractions of biochar decreases while other fractions increases as the pyrolysis temperature increases. The NMR technique identified hydroxyl apatite and dehydrated calcium phosphate in PLC; and the content and crystalization level increased as the pyrolysis temperature increased.